# Why didn't the glass break?

In the figure below, a needle has been placed in each end of a broomstick, the tips of the needles resting on the edges of filled wine glasses. The experimenter strikes the broomstick a swift and sturdy blow with a stout rod. The broomstick breaks and falls to the floor but the wine glasses remain in place and no wine is spilled. This impressive parlor stunt was popular at the end of the nineteenth century.

What is the physics behind it?

## 4 Answers

This is similar to the classic example in Halliday and Resnick (or at least, it was in the Great Eggplant version), where there are two sections of rope, identical material, one tied to the ceiling, then to a mass, and the other hanging from the mass.

If you pull slowly and firmly, the combination of your pull and the gravitational force $Mass*g$ will cause the upper rope to break first. If you pull with a sudden strong jerk, the lower rope will break because it can't withstand the peak force.

EDIT: here's a photo of an experimental setup to accompany that H&R discussion: weights and ropes . As Owen B mentioned, inertia plays a key role here. Backtrack from that link to find the section of H&R of interest.

My money's on the same thing in your case: the rod shatters before it can transmit the force of the blow to the glasses.

• I think you've got it, though you might like to put the word inertia in your answer somewhere. Taking the experiment to its limits; if we simply lobbed the broomstick up into the air and then whacked it really fast while it was at the top of its trajectory, we would get the same result - even though the stick was not being supported by anything! – Oscar Bravo Dec 22 '15 at 14:45

If you strike the broomstick sharply enough, the ends of the broomstick actually move up, not down. When the broomstick breaks, each broomstick piece rotates about the center of its mass, which is in the middle of that piece. So for each piece, the end at the breaking point moves downward, but upward on the other end.

• Also, the broomstick might be partially cut through in the middle. Then this method would be more effective. – James Dec 22 '15 at 12:49
• I strongly disagree with the "bends upward" claim. There's no force in the system to counteract gravity. – Carl Witthoft Dec 22 '15 at 13:24
• @Carl, imagine a single stick floating in space. If we tap one end perpendicular to the stick, the stick will rotate. – Digiproc Dec 22 '15 at 14:19
• @Digiproc Yes, but not when you tap it near the middle, not to mention when there is gravity involved. – Carl Witthoft Dec 22 '15 at 14:49
• @CarlWitthoft: See the video at this website to see the rotation in slow motion. physicsforce.com/demo-details-glasses-stick – James Dec 22 '15 at 14:59

Well, one part of the answer is undoubtedly the fact that wine glasses, or indeed glass bottles, are actually extremely strong. The fact they are also extremely brittle tends to hide this fact. So the pressure being exerted on the glass itself is obviously not enough to break it. This is due to the major pressure being the centre of the broom stick which acts as a kind of reverse lever. It also bends and hence limits the shock transmitted to the glasses.

• I don't think so. It's a matter of shock. – Carl Witthoft Dec 22 '15 at 13:24
• The shock is mitigated by the bending of the wood – user56903 Dec 22 '15 at 13:34
• If this were true, you could break the wood by slowing pressing on it. But that ain't so. The crucial thing is speed and that always means that inertia is at work... – Oscar Bravo Dec 22 '15 at 14:49
• Actually, it might be so. As I said, glass is very strong. – user56903 Dec 22 '15 at 14:50

when a person hits a broomstick, the direction of the force F applied is vertically downwards. according to 3rd law of Newton, there would be an equal and opposite reaction. so in this case the broomstick breaks into two parts, each part having a torque equal to the force F and moment arm (1/2 length of the broomstick)thus not touching the wine glasses as the torque tends to rotate the broken pieces away from the glasses. and lastly the broken pieces fall down with some precise angle i.e produced by torque.